The time synchronization in communication systems is to coordinate the time of communication equipment with the universal time, also called phase synchronization.
A communication network, especially wireless devices, requires the synchronization among communication systems. Table 1 lists the specific requirements of wireless devices on the synchronization:
TABLE 1WirelessRequirement on Clock FrequencyRequirement on TimeTechnologyAccuracySynchronizationGSM0.05 ppmNAWCDMA0.05 ppmNACDMA20000.05 ppm  3 usTD-SCDMA0.05 ppm1.5 usWiMax0.05 ppm  1 usLTE0.05 ppmInclined to adopt timesynchronization
The Passive Optical Network (PON) technology is a point to multipoint optical access technology, and a PON consists of an Optical Line Terminal (OLT) on the office side, an Optical Network Unit (ONU) on the user side and an Optical Distribution Network (ODN). The PON has multiple types, and can be roughly divided into a wavelength division PON, a power PON and a hybrid PON combined by the two types above. The power PON is further divided into an ATM (asynchronous transfer mode) Passive Optical Network (APON), a Gigabit Passive Optical Network (GPON) and an Ethernet Passive Optical Network (EPON) based on different protocols of link layers.
The power PON generally adopts a Time Division Multiplex (TDM) broadcast mode and a Time Division Multiple Access (TDMA) mode for the downlink and uplink thereof respectively, and can flexibly constitute tree, star and bus and other topological structures (typically, the tree structure).
The so-called hybrid PON refers to that multiple power PONs are available in an ODN, and adopt different wavelengths.
The PON can be used as a bearer network in an access section. FIG. 1 illustrates a diagram of networking and clock transfer of a PON in the field of mobile communications, and typical networking of the PON in the mobile communications is shown as FIG. 1. It is needed for the PON to complete the transfer of two tasks: service stream transfer and clock synchronization transfer, and FIG. 1 shows the transfer and distribution mode of a clock stream.
The PON protocol only realizes the frequency synchronization per se, i.e. the synchronization of an ONU to an OLT; however, the phase difference between ONUs is random, i.e. incapable of supporting the time synchronization, thereby not realizing the mobile communications or the network utilization that requires the time synchronization.
IEEE1588 can realize the time synchronization among network devices that
1) based on the assumption of network characteristic symmetry, a master and slave mode is used to tag a timestamp on a packet, and periodically distribute a clock, and a receiver measures the clock offset and the delay;
2) a point-to-point link can provide the highest accuracy, and a boundary clock is introduced, independent of delay jitter, and can reach the time accuracy of 10 ns every hop; and
3) the frequency synchronization and the time synchronization can be realized.
The following describes the detailed implementation mechanism, and FIG. 2 illustrates a diagram of the implementation mechanism of the IEEE1588 protocol.
1) Provided that the time difference between a Master device and a Slave device is “Offset”;
2) the Master device sends a synchronization packet with a timestamp to the Slave device at the time of T1, and the Slave device locally receives a synchronization message at the time of T2; the following equation can be established, wherein the “Delay” is the transmission delay of the synchronization message from the Master device to the Slave device;T2=T1+Delay+Offset  (1)
3) the Master device sends a Follow_up message to the Slave device;
4) the Slave device sends a Delay_Req (delay request) message to the Master device at the local time of T3;
5) the Master device sends a Delay_Resp (delay response) message to the Slave device at the local time of T4; the following equation can be established:T4=T3+Delay−Offset  (2)
6) the value of the “Offset” can be calculated by combining Equation (1) with Equation (2);
7) if the Sync message, Follow_up message, Delay_Req message and the Delay_Resp message can be periodically sent between the Master and Slave devices, the value of the “Offset” can be dynamically updated, thereby keeping the Slave device synchronized to the Master device in time; and
8) a time accuracy of 10 ns can be obtained between the Master and Slave devices.
From the above, it can be known that the IEEE1588 protocol is based on an assumption of the network characteristic symmetry, i.e. the delay and jitter from the Master device to the Slave device is the same with those from the Slave device to the Master device, and the accuracy of time synchronization is directly depended on the jitter magnitude of a link.
The GPON (i.e. a PON defined by the ITU G,984.1-4 specifications) bearing an Ethernet service refers to that the GPON bears a protocol stack of an Ethernet protocol packet, as shown in FIG. 3. To improve the bearing efficiency, a GPON encapsulation method (GEM) layer can segment and recombine packets for an upper layer service data stream, so that a message of the upper layer data stream is possibly segmented into multiple parts, each of which is sent at unfixed and different time, and a receiving terminal detects that whether all the segmented parts of a data packet have arrived, and, if so, implements an encapsulation to recover a complete upper layer data message. However, for the upper layer data message, the GPON is a delay-varying network, i.e. the jitter thereof is large, so that the IEEE1588 protocol cannot run on the GPON directly. Therefore, the accurate time synchronization of the ONU with the OLT cannot be guaranteed in the PON.